This is an ancient urban myth, I remember it from the early 90's at college, friend of a friend etc. Back then it was one of the student nurses who had left years before, always someone who can't deny or confirm. I imagine it came about as a what if, "I pulled a bloke and had to go for a dump, and it wouldn't flush!", friends laugh and then inject possible scenarios to make things worse. One of them repeats it as gospel and it goes from there.
Thing is though you'd just use the bag to push it around the bend. Crisis averted.
On the other hand, I did know someone who bought a plant in the UK that came with a Tarantula in it, it was dead (or may have been a shed skin), but it did happen. Another urban myth, which I know, in part at least, to be true.